This invention deals with voice encoding techniques and more particularly with such techniques enabling multirate encoding of voice signals.
Sub-band coding (SBC) associated with dynamic allocation of bits resources (DAB) has been shown to be an efficient approach for digitally encoding speech signals. Such coding methods have been extensively described in the literature and patents. One may mention U.S. Pat. No. 4,142,071; the article "16K bps Real Time QMF Sub Band Coder Implementation", in IEEE ICASSP, Denver, April 1980, by C. Galand and D. Esteban; and the article "32K bps CCITT compatible Split Band Coding" presented by D. Esteban and C. Galand at the 1978 IEEE International Conference held in Tulsa, Oklahoma. These references should be considered herein incorporated by reference.
According to these coding methods, at least a portion of the original speech bandwidth is split into several sub bands the contents of which are quantized with a dynamic sharing of the quantizing resources throughout the sub bands based on their respective energy contents. In addition, applying Block Companded Pulse Code Modulation (BCPCM) to the above methods enables dynamically allocating the quantizing resources over fixed length consecutive speech segments (e.g. 16 ms long). In other words, the sub band quantizing resources distribution needs only being repeated at the segment rate, i.e. every 16 ms. By quantizing resources one usually means quantizing bits as disclosed in the above cited references.
For more detailed information on BCPCM one may refer to an article by A. Croisier relating to a lecture given at the International Seminar on digital Communication, in Zurich, Switzerland, and entitled, "Progress in PCM and Delta Modulation: Block Companded Coding of Speech Signal". In summary, for each block of N samples representing a 16 ms long segment of voice signal, a scale factor C is chosen such that the largest sample of the block does not fall outside the coding limits. Them, the N samples of the block are requantized, based on this factor.
In Split band coders this requantizing is performed separately in each sub band, with the quantizing resources being dynamically allocated throughout the sub bands based on their energy contents. BCPCM suits particularly well for coding methods requiring information relative to energy of the signal to be processed since energy could be conveniently approximated using a readily available BCPCM parameter, i.e. the so-called scale factor or characteristic "C".
In addition, application of sub-band coders to Time Assignment Speech Interpolation systems (TASI) has been proposed in the European patent application No. 85 820 "Multirate digital transmission method and device for implementing said method" with a new TASI approach for embedded bit stream. For that purpose, the output of the SBC speech compressor is arranged in an embedded bit stream, i.e. which accepts bit deletion and insertion for dynamic rate conversion. This property is of high importance in a digital communication network, since it allows the bit stream to be flagged at any overloaded node without tandeming or freeze-out.
On the other hand, sub-band coding with dynamic allocation of levels (DAL) has been shown to achieve a significant improvement over SBC with dynamic allocation of bits (DAB), specially for low bit rates. Such a technique has been disclosed in the European application No. 86,109,498.5 dated July 11, 1986 herein incorporated by reference.